1. Field of the Invention
This invention relates to quasi-optic grid arrays, and in particular to tunable and modulatable grid oscillators.
2. Description of Related Art
Broadband communications, radar and other imaging systems require the transmission of radio frequency (xe2x80x9cRFxe2x80x9d) signals in the microwave and millimeter wave bands. In order to efficiently achieve the levels of output transmission power needed for many applications at these high frequencies, a technique called xe2x80x9cpower combiningxe2x80x9d has been employed, whereby the output power of individual components are coupled, or combined, thereby creating a single power output that is greater than an individual component can supply. Conventionally, power combining has used resonant waveguide cavities or transmission-line feed networks. These approaches, however, have a number of shortcomings that become especially apparent at higher frequencies. First, conductor losses in the waveguide walls or transmission lines tend to increase with frequency, eventually limiting the combining efficiency. Second, these resonant waveguide cavities or transmission-line combiners become increasingly difficult to machine as the wavelength gets smaller. Third, in waveguide systems, each device often must be inserted and tuned manually. This is labor-intensive and only practical for a relatively small number of devices.
Several years ago, spatial power combining using xe2x80x9cquasi-opticsxe2x80x9d was proposed as a potential solution to these problems. The theory was that an array of microwave or millimeter-wave solid state sources placed in a resonator could synchronize to the same frequency and phase, and their outputs would combine in free space, minimizing conductor losses. Furthermore, a planar array could be fabricated monolithically and at shorter wavelengths, thereby enabling potentially thousands of devices to be incorporated on a single wafer.
Since then, numerous quasi-optical devices have been developed, including detectors, multipliers, mixers, and phase shifters. These passive devices continue to be the subject of ongoing research. Over the past few years, however, active quasi-optical devices, namely oscillators and amplifiers, have evolved. One benefit of spatial power combining (over other methods) using quasi-optics is that the output power scales linearly with chip area. Thus, the field of active quasi-optics has attracted considerable attention in a short time, and the growth of the field has been explosive.
It is believed that the first quasi-optical grid array amplifier was developed by M. Kim et al. at the California Institute of Technology. This grid used 25 MESFET differential pairs, demonstrating a gain of 11 dB at 3 GHz. As shown in FIG. 1, a typical grid amplifier 10 is an array of closely-spaced differential pairs of transistors 14 on an active grid 12 sandwiched between an input and output polarizer, 18, 24. An input signal 16 passes through the horizontally polarized input polarizer 18 and creates an input beam incident from the left that excites rf currents on the horizontally polarized input antennas 20 of the grid 12. These currents drive the inputs of the transistor pair 14 in the differential mode. The output currents are redirected along the grid""s vertically polarized antennas 22, producing a vertically polarized output beam 30 via an output polarizer 24 to the right.
The cross-polarized input and output affords two important advantages. First, it provides good input-output isolation, reducing the potential for spurious feedback oscillations. Second, the amplifier""s input and output circuits can be independently tuned using metal-strip polarizers, which also confine the beam to the forward direction. Numerous grid amplifiers have since been developed and have proven thus far to have great promise for both military and commercial RF applications and particularly for high frequency, broadband systems that require significant output power levels (e.g.  greater than 5 watts) in a small, preferably monolithic, package. Moreover, a resonator can be used to provide feedback to couple the active devices to form a high power oscillator.
One non-modulatable source configuration, known as a xe2x80x9cKim oscillator,xe2x80x9d is described in M. Kim, E. A. Sovero, J. B. Hacker, M. P. De Lisio, J. J. Rosenberg, D. B. Rutledge, xe2x80x9cA 6.5 GHz-11.5 GHz Source Using a Grid Amplifier with a Twist Reflector,xe2x80x9d IEEE Trans. on Microwave Theory and Tech., Vol. 41, No. 10, pp. 1772-1774, October, 1993. The basic concept of this oscillator is to apply external feedback to a grid amplifier in order to induce it to oscillate. FIG. 2A shows a functional schematic of the standard Kim oscillator disclosed therein. FIG. 2B shows an exploded view of the physical configuration of a standard Kim oscillator. As seen, a twist reflector 40, comprising a tilted polarizer 42 and a mirror 44 rotates the y-polarization of the portion of the amplified output beam from the amplifier 46 that is incident upon it and reflects it back into the input of grid amplifier. The polarizer 42 of the twist reflector 40 can be treated as a perfect reflector to energy polarized along its wires and as invisible to energy polarized perpendicular to them. The grid amplifier active array provides gain and gain compression. The frequency selectivity (tuning) in the feedback is accomplished by a phase delay 41, which is primarily set by the physical separation xe2x80x9cdxe2x80x9d of the twist reflector and the grid array. The output polarizer 48 provides both isolation of the output from the input polarization as well as providing impedance matching for the input polarization.
Another type of grid oscillator is a voltage controlled quasi-optical oscillator, disclosed by T. Mader, S. Bundy, Z. B. Popovic, xe2x80x9cQuasi-Optical VCOs,xe2x80x9d IEEE Trans. on Microwave Theory and Tech., Vol. 41, No. 10, pp. 1775-1781, October, 1993. Unfortunately, this VCO, while tunable, produces relatively little output power and has a fairly narrow tuning range.
Grid oscillators that include an external twist-reflector feedback network (xe2x80x9cKim oscillatorsxe2x80x9d) are conventionally frequency tuned by mechanically translating the feedback network along the axis of the grid""s transmitted beam. Unfortunately, the need to physically move the reflector to tune the device is undesirable for numerous reasons. The addition of translation means adds bulk and expense to these otherwise highly reproducible structures. Mechanical tuning can be imprecise and slow. Further, the oscillator cannot modulate its output signal. Thus, such devices are impractical for most applications.
Electronic tuning of grid oscillators having twist reflectors has been proposed as a possible solution. One solution entails introducing a varactor-loaded tilted filter structure in place of the tilted polarizer in the twist reflector. This solution should allow direct phase modulation of the grid oscillator, but requires a specialized processing unit to generate the modulation signals to be applied to the varactors, and a specialized high-speed amplifier to actually drive the varactors. These extra components can add cost and complexity to the oscillator. Thus, there is a definite need for a tunable and modulatable grid oscillator that does not require such specialized circuitry.
The present invention, which addresses this need resides in an electronically tunable and modulatable quasi-optic power signal source. The power signal source includes a quasi-optic grid array oscillator having an output that sources an output signal, and means for introducing a signal to entrain the frequency and phase of the oscillator output to a reference signal having a frequency and phase that are specified as a function of time, such that the oscillator output bears a predetermined relationship to the frequency and phase of the reference signal. The grid array oscillator includes a grid amplifier having an input and output and a twist reflector spaced apart from the amplifier by a predetermined distance. In a preferred embodiment, the means for entraining is an input port associated with the oscillator that accepts an external injection-locking reference signal having a frequency and phase that are specified as a function of time. This electronic phase modulation described by this invention allows one to utilize signals that are typically generated with conventional transmitter systems. These phase-modulated signals at either the transmission frequency (or an integral fraction thereof), are usually generated at the output of an upconverter and applied to a preamplifier (or amplifying frequency multiplier) before the final power amplification stage. This invention can be deigned into a transmitter system, thereby eliminating the need for the preamplifier (or frequency multiplier). This allows a reduction in part-count and simplification of the overall transmitter system.
The input port of the present invention may be located at any one of a number of locations in the oscillator. In one preferred embodiment, the input port is located at the twist reflector. More specifically, the twist reflector includes a mirror and an angled polarizer associated with the mirror and the input port is located within the mirror of the twist reflector. In another embodiment of the power signal source of the present invention, the input port is located at the input of the grid amplifier. In yet another embodiment, the input port is located at the output of the grid amplifier. In still yet another embodiment, the input port is located at the output of the oscillator.
The reference signal source that is applied to the reference port may take several different forms to achieve the same oscillator output. For example, in, one embodiment, the output of the oscillator tracks the frequency and phase of the reference signal in a one to one relationship. That is, the frequency and phase of the oscillator output is substantially the same as the frequency and phase of the input reference signal at the reference port. Alternatively, the frequency and phase of the oscillator""s output may be a multiple of the frequency and phase of the reference signal.
In another aspect of the present invention, an electronically tunable and modulatable quasi-optic power signal source is disclosed. The source includes a quasi-optic grid array oscillator that includes a grid amplifier, a polarizing feedback structure and an output that sources an output signal, and a reference input port associated with the oscillator that accepts an external injection-locking reference signal having a frequency and phase that are specified as a function of time such that the oscillator output bears a predetermined relationship to the frequency and phase of the reference signal.
The oscillator of the present invention may also serves an a novel amplitude modulator. In one aspect, the electronically tunable and modulatable power signal source includes a quasi-optic grid oscillator having an output that sources an output signal, a reference input port and a controller that regulates the amplitude of the output signal of the oscillator. The reference port may be located at any appropriate location on the oscillator. The port accepts an external injection-locking reference signal having a frequency and phase that are specified as a function of time such that the oscillator output bears a predetermined relationship to the frequency and phase of the reference signal. In one aspect of this invention, the means for controlling the amplitude of the output is an external voltage modulator connected to the oscillator that directly modulates of the control bias voltage of the oscillator.
In another aspect of the invention, a system for electronically tuning and modulating a power signal source is disclosed. The system includes two quasi-optic grid oscillators each having an output that sources an output signal, where the output signals are summed together. Also included is outphasing modulation control unit that supplies each of the grid oscillators with its own phase-modulated reference signal. Using this technique, called Chireix""s technique, the phasor sum of the oscillators outputs a desired phase and amplitude modulated signal. In one embodiment, the outputs of the two oscillators are summed in free-space. In another, the outputs of the two oscillators are summed using guided-wave power combining.
The two amplitude modulation techniques described above may also be combined to result in a more efficient and accurate quasi-optic AM modulatable oscillator. In particular, dual oscillator system described above that employs Chireix""s outphasing modulation technique may also employ direct amplitude modulation to each oscillator, such that the amplitude of the first oscillator output is directly modulated with a first external voltage modulator and the amplitude of the second oscillator output is directly modulated with a second external voltage modulator.
A method of electronically tuning and modulating a quasi-optic grid oscillator having a signal output is disclosed. The method includes applying an entraining reference signal to the oscillator and adjusting the reference signal to a desired frequency and phase such that the oscillator output bears a predetermined relationship to the frequency and phase of the reference signal.